Follow these steps to add a collaborator to your repository.
Type in the user name of the person you want to invite as a collaborator. As you type the name, a list will be auto-populated. Once you type in enough of the name, you can click the user name in the list.
Once you find the name of the person your want to add as the Collaborator, click "Add (username) to this repository"
The collaborator will receive an email and can accept the invitation with a link in the email.
OR, from the collaborator account, navigate to the repository page, with
invitations
added to the URL. It will look like this:
https://github.com/OWNERNAME/REPONAME/invitations
From here, accept the invitation to collaborate
Go back to the "Manage Access" page and you should be able to see that the collaborator has been added.